(a) Field of the Invention
The present invention relates to a reduced beat noise microwave amplifier and, more particularly, to a microwave amplifier for amplifying microwave signal including a plurality of different carrier frequencies.
(b) Description of the Related Art
In general, a satellite communication system uses microwave (very-high-frequency wave) carrier modulated with relatively low frequency input signal. In the transmitter of the satellite communication system, the microwave signal is amplified with a high gain using a microwave amplifier including an active elements implemented by FETs. In the conventional microwave amplifier, a suitable DC bias voltage is applied to the active elements through a micro-strip line or xcexg/4 line which acts as a high impedance element against the carrier frequencies in the microwave signal. xe2x80x9cxcexgxe2x80x9d means the wavelength of the microwave signal.
FIG. 1 shows a circuit diagram of a portion of a microwave amplifier, and FIG. 2 is a top plan view of the circuit portion of FIG. 1. FET 10 has a drain connected to a terminal of xcexg/4 line 11, the other terminal of which is connected to a DC source line for applying a bias voltage VDS and to the ground through a capacitor 12 which passes low frequency components.
The xcexg/4 line 11 and the capacitor function as a DC bias circuit for applying the DC bias voltage VDS to the drain of the FET 10, which has a high impedance against the carrier frequencies. Such a configuration is described in Patent Publication JP-B-2-61175, for example.
In the conventional DC bias circuit as described above, although the xcexg/4 line 11 has a low resistance component R and advantageously provides a low voltage drop across the xcexg/4 line 11, the reactance component jX in the xcexg/4 line is not considered in connection with low frequency beat components when the microwave signal includes a plurality of carrier waves. A combination of the low frequency beat and the reactance component jX of the DC bias circuit in the beat frequency range generates a significant voltage drop in the bias voltage VDS.
For example, when two carrier signals having carrier frequencies f1 and f2 (f1 less than f2) are mixed, a beat component is generated having a frequency of f2-f1, which causes a fluctuation in the bias current supplied to the drain of the FET 10.
The xcexg/4 line passing the bias current has an impedance R+jX, which provides a drain voltage VDS(t) to the FET 10 as follows:
xe2x80x83VDS(t)=VDSxe2x88x92ID(t)xc3x97(R+jX)xe2x80x83xe2x80x83(1),
Wherein ID(t) is the current passing the thorough the xcexg/4 line to the drain.
If a beat component is generated, the drain voltage VDS(t) of the FET 10 fluctuates based on the ID(t), the resistance component R and the reactance component jX, according to the equation (1). It is found by the inventor that the output of the microwave amplifier involves such a distortion due to the fluctuation in the DC bias voltage caused by the beat component.
It is an object of the present invention to provide a microwave amplifier capable of reducing a signal distortion caused by the beat component which is generated from a plurality of carrier waves.
The present invention is directed to a microwave amplifier includes an active element such as FET for receiving an input microwave signal including a plurality of carrier waves having carrier frequencies through an input terminal of the active element to output an amplified signal through an output terminal of the active element. The microwave amplifier of the present invention includes a filter block having first and second terminals, the first terminal being connected to one of the input terminal and the output terminal, and at least one first capacitor connected between the second terminal and a ground line, the filter block exhibiting a first impedance at the carrier frequencies and a second impedance at a beat frequency generated from the carrier waves, the first impedance being higher than the second impedance.
In accordance with the microwave amplifier of the present invention, the filter block and the capacitor pass therethrough the beat component generated by a plurality of carrier waves to the ground while maintaining the microwave signals, thereby reducing the signal distortion based on the beat component.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.